In ballpoint pens using high-viscosity oil-base ink, a mixture of a mineral oil and a metallic soap (i.e., grease) has been used as an ink follower composition also called "liquid stopper" or "a back flow preventing composition".
When applied to ballpoint pens using middle-viscosity oil-base ink or shear-thinning aqueous ink, the ink follower applicable to the high-viscosity oil-base ink does not follow the ink satisfactorily or fails to prevent the ink from flowing backward because of its large viscosity change with temperature. Hence, various gelatinous polybutene-based ink followers comprising a hardly volatile organic liquid, typically exemplified by liquid polybutene, and a thickener have been proposed and put to practical use broadly in ballpoint pens using the aforementioned ink.
Conventional thickeners include dibenzylidene sorbitol, tribenzylidene sorbitol (see JP-B-1-10554), N-acylamino acid derivatives (see JP-A-57-200472), clay having been surface-treated with an onium compound having a long-chain alkyl group (see JP-B-7-17872 corresponding to U.S. Pat. No. 4,671,691), and silica having been surface-treated with a hydrophobic group (see JP-B-6-33025). The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.
The ink follower comprising the hardly volatile liquid and the conventional thickener is used to plug the end of ink contained in a transparent or semitransparent ink tube so as to prevent ink from evaporating and from flowing backward even when the ballpoint pen is left to stand for a long period of time with its tip upward or when a shock is given to the ballpoint pen by, for example, dropping. However, when the ink follower follows the ink with the ink consumption, it moves with the ink adhering to the inner wall of the ink container. As a result, a certain amount of the ink remains on the inner wall uselessly so that the writing length is shorter than what should have been expected from the amount of the ink in the container. Besides, the ink staining the inner wall not only makes the remainder of the ink invisible but deteriorates the appearance.